Permanent magnet electric machines, such as motors and generators, include a stationary stator that defines salient poles and inter-pole slots that are located between the salient poles. The stator is often mounted on an inner surface of a machine housing with the salient poles projecting radially inwardly. The permanent magnet electric machines also include a rotor that is mounted on a shaft, that includes rotor poles and that rotates on the shaft relative to the stator. The rotor poles include permanent magnets that are attached to a radially outer surface of the rotor. Winding wire is wound around the stator poles in the stator slots. A circuit board or another connection device couples the stator pole windings to a drive circuit. The drive circuit generates a set of stator winding currents that are output to the stator pole windings and that result in a rotating magnetic field. The rotating magnetic field in the stator poles attracts the poles of the rotor to cause the rotor to rotate.
Electric machines with permanent magnet rotors often have cogging torque that adversely impacts machine performance. Cogging torque is caused by the variation of magnetic permeance as seen by a rotor pole as it passes the stator poles and the slot openings. Cogging torque occurs when the stator windings are un-energized. The rotor seeks a rotational position that results in the lowest magnetic circuit reluctance (or the highest permeance). The rotational position with the lowest magnetic circuit reluctance occurs when a rotor pole is aligned with a stator pole. When the rotor pole is aligned with a slot opening, the rotor pole will attempt to align itself with a stator pole, thereby producing torque. The cogging torque oscillates between positive and negative torque, depending on the position of the rotor poles with respect to the stator poles. The torque oscillations cause vibration and noise within the permanent magnet electric machine. The variation in torque can also cause vibration in the equipment that is driven by the machine, which causes additional noise.
Various methods for reducing cogging torque have been proposed. In one method, the permanent magnets are skewed in an angled pattern or in a herringbone pattern on the outer surface of the rotor. Skewing the permanent magnets increases material and manufacturing costs due to the complex and non-uniform shape of the permanent magnets. The non-uniform permanent magnets are also difficult to assemble.
Therefore, a permanent magnet electric machine that significantly reduces cogging torque and that can be assembled relatively easily and with relatively low manufacturing costs would be desirable.